a question about spaceplanes

[Cirocco]

Hi guys,

Recently I have once again caught the space plane bug, but I have come across a bit of an issue with my space planes/SSTO’s and I’m wondering if you guys can help me out. I have no real problems getting my ships into orbit, but I always feel I’m doing it terribly inefficiently because I always seem to waste so much fuel and oxidizer on the insertion burn. I really think the biggest area for improvement is the height at which the air breathing engines give out: I can’t seem to get anything meaningful above 20km. This also means I’m limited to getting around 1000m/s horizontal velocity on air breathing engines.

So I was wondering: what can you do to raise that height? I know about the whole stacking a ton of intakes on top of each other, but that feels like cheating to me. Have you guys come up with other methods of making space planes more efficient? Be it a clever way of designing to add more intakes without stacking them, a really good ascent profile, clever use of nuclear engines or anything else: what do you guys do to make your space planes/SSTO’s as efficient as possible?

Thanks in advance

Cirocco

[Laie]

First off: if you engines flame out at 20km, you've probably got only one intake per engine. You should at least consider the 1-to-4 adapters on wing-mounted engines, that will last you until 25-26km or so.

Spaceplanes work so well because jets are really efficient: the same fuel will give you about 3x as much thrust when you burn it in a jet rather than a rocket motor. Jets being comparatively overpowered and needing no oxidizer also helps, of course, but it's the 1200-ish Isp that really makes your day. Jets would still offer a good deal if they were three times as heavy as they currently are.

Rule 1: Any speed gained from jets is cheap speed.

Rule 2: The higher you go, the faster you can get. The atmosphere gets thinner quickly past 20km, and ever more quickly the higher you get. Consider this graph (man, how do I thumbnail these things?):

You may use "terminal velocity" as a proxy for "relative drag at altitude". If a given amount of thrust will get you to a certain airspeed at 20km, the same amount of thrust will make you go about twice as fast at 25km, and three times as fast at 30km. If you have a large, many-jet vessel, you may notice that you can still gain speed & climbrate even after switching off the first jets. That's because the reduced drag at high altitudes more than offsets a 10% loss in thrust.

What I'm trying to get at: no matter how high your jets take you, there's always a tremendous benefit to operating them just a little longer. Unfortunately, the ability to do so depends solely on the number of intakes you manage to attach to your plane.

Edited May 26, 2014 by Laie
full rewrite

[thescientist]

If you don't like intake spamming, you might use your jet as well as possible. As usual, experience helps you, but there are some way to squeeze all the power your engines can give. Here there are some:

1. Throttle down as you get higher (and faster). Your engines should work for some extra km without flaming out, because reducing the throttle also reduce the required fuel/air to keep them going.

2. Steep ascent are good only for takeoff. Keeping your vertical speed low as well as your AoA should let you go faster.

3. Keep the intake straight. If they aren't, the intake air will fall resulting in flameout.

4. Use a smart design. Remember that a big plane needs more fuel and engines that a small one. Big spaceplanes might be very inefficient at getting high speed without falling apart.

5. Use ram intake (because they have an higher intake area with a small physical footprint) and bicoupler to increase intake without "spamming" them.

[Pecan]

The others have covered all the main points. I'd just add that you have to make sure you're climbing slowly, if at all, once in the area your engines start to run out of air. The important thing is to build horizontal, not vertical, speed. The faster you go the more air you get and the higher you can go, slowly. As a rule I aim to start levelling-out at 15km, building horizontal speed to 1km/s at 20km and 100m/s faster every km above that (1,100m/s at 21km, 1,200m/s at 22km, etc.) The big trick that got me into orbit with planes was learning to throttle-back, as thescientist mentions. Once the engines get near to flameout you can keep them alive and still thrusting that way - even with reduced thrust they will still accelerate your aircraft for a while. Repeat as required until you're not accelerating any more, then switch to rockets. With 1 intake per engine it's hard but 2 should not look like spamming and will take you to ~1,800m/s and 28km. As orbital speed is ~2,200m/s that leaves you 400m/s to make-up with rockets. More intakes per engine makes it easier to reach higher speeds, up to about 4 can be done in an aesthetic way; I usually use tri- or quad-couplers. You also have to consider their drag though - if it is ahead of the CoM it can make the plane impossible to control at high altitude/speed either on launch or re-entry.

[O-Doc]

I would add that, when you throttle down your turbo/s at some point(common TWR is half throttle for NERVA or about quarter for T-series) engage your rocket engine and continue to throttle down to make use of all of the air available. Once the jets finally choke or you stop gaining surface speed, cut your turbos and power back up for for sub-orbital hop which should be very close(within 50m/s) to orbital speed at your apogee.

I have a plane in my catalogue called the Crossbow which is only two radial intakes and a tiny rocket but, has enough dV to land safely on Mun for a splash and dash back home.

[LaytheAerospace]

1. Increase your intake:engine ratio. 2:1 or 3:1 will give you a better cruise ceiling. You really want to cruise at around 25km to build speed. Higher is better, but diminishing returns have already kicked in and you need something like 8:1 to cruise at 33km.
   
2. Climb as fast as possible to your cruise ceiling, then level out and build speed as long as you can stand. I consider ~1400 m/s my minimum speed before nosing up for the orbit burn. 2000+ is totally doable if you're patient.
   
3. Get MechJeb and have it manage your intakes and prevent jet flameout. Make a custom window that has your max intake air with all intakes open, current intake air and intake air needed max. When Your max intake air is greater than your current intake air, it means you have too many intakes for the altitude you are currently at. If they are the same, it means you have exactly as many intakes as needed for your current altitude. When they drop below intake air needed max, you no longer have enough intakes. You want to keep these three numbers as close to each other as possible, which will be the highest point your intakes can take you before throttling kicks in.
   

[Laie]

1. 
   
2. Climb as fast as possible to your cruise ceiling, then level out and build speed as long as you can stand.
   

I disagree. In my experience (which is limited to rather heavy craft, so ymmv) it is wholly sufficient to gradually ease the climb rate, crossing the last 2000m at about 15-20m/s (i.e. in about two minutes). Long speed-building at/near the ideal cruising altitude doesn't lead to me having any more oxidizer left by the time I am in a circular orbit. Many vessels can climb through the ideal zone even a little faster without wasting fuel.

[Pecan]

I disagree with your disagreement; 1 or 2 km altitude makes practically no difference whatsoever, 100 or 200m/s does.

[Cirocco]

Allright, thanks for the answers guys. I re-designed my orbital bus yesterday to have a 2:1 intake to engine ratio and she runs better now. I can climb to about 25km and 1400m/s surface speed. She also now can carry 8 kerbals and 3 pilots (2 optional) rather than the original 8+1.

As for ascent profile: I agree with the more gradual climb because of one simple reason: ISP of a jet engine is at max at 5km. So I do a mad dash for 5km, then get up to speed (around 180m/s) and then pull into about 45° climb where I feel The height gain and surface spped gain go at about the same rate. I start leveling off between 10 - 13km and then shallow climb to increase speed.

Guess I'll see if I can slap on some extra intakes without messing with the center of mass too much. One or two structural hull segments+bi-coupler+two intakes shouldn't weight too much. Just hope the off-set drag won't **** me over at high altitude.